Indicating plug fuse



March 13, 1934. C, LA MAR 1,951,307

INDICATING PLUG FUSE Filed July 30, 1932 i iin v 33m INVENTOR- Patented Mar. 13, 1934 UNITED STATESI PATENT QFFICE Claims.

This invention relates to electrical devices, particularly to thermal fuses for use in connection with electrical circuits, and more especially to what is known as a plug fuse, combined with I means for indicating the integrity of the fusible element.

It is quite universally known that it is necessary to protect electrical circuits by means of devices, such as thermalfuses, which melt or become disintegrated or destroyed when an excessive now of current passes through the circuit of which the fuse forms a part. An especially well known form of thermal fuse is that known as plug fuses. Ordinary plug fuses consist, generally, of an insulating body having two terminals on the outside thereof so constructed and arranged as to be readily brought in contact with the terminals of an electrical circuit, by way of what is known usually as a fuse block. These fuses are universally used; are made by many different manufacturers; and must all be usable in practically any of the well known forms of fuse block;

so that they have become, in truth, standard, and may well be referred lto as, standard fuse plugs. At any rate they are substantially standr ard as regards their general external dimensions.

Various forms of plug fuses have been made, especially for the purpose of enabling one to rapidly discern whether or notl the fusible or thermal element is intact. It is quite the universal impression that success has not been attained in regard to the feature of indicating the integrity ofthe fusible element.

A principal object of applicants invention is to provide a device, which may well be called a combined fuse and fuse integrity indicating device. In providing such a device, applicant has had in mind that it preferably should be in external dimensions substantially the size of the standard plug fuse, so that it may be used with a facility equal to that of a plug fuse in those situations Where a plug fuse is now used.

A further object which applicant has in mind in producing his combined fuse and fuse integrity indicator is that the cost must be reduced to a point where, if possible, it approximates the present cost of ordinary plug fuses. To this end, applicant has, as a further object of his invention, the production of a device in two parts, separable one from the other, one part performing the functions now performed by the ordinary plug fus'e, and the other part performing the indicating functions desired byapplicant.

A further object of applicants invention is to 55 produces. combined fuse and fuse integrity .inm

dicator which will be substantially the same external dimensions as a standard plug fuse.

Other objects and advantages will appear as the description of the particular physical embodiments selected to illustrate the invention pro- 00 gresses, and the novel features will be particularly pointed out in the appended claims.

' In describing the invention in detail and the particularphysical embodiments selectedto illustrate the invention reference will be had to the accompanying drawing and the several views thereon, in which like characters of reference designate like parts throughout the several views, and in which:

Figure 1 is a side elevational view of a plug 70 fuse embodying applicants invention; Fig. 2 is a sectional View of the device as shown by Fig. 1; Fig. 3 is a. sectional View of the device, as shown by Figs. 1 and 2, on the plane 'indicated by the line III-III of Fig. 2, viewed in th'e direction of 75 the arrows at the ends of the line; Figs, 4, 5, 6, 7,

8, 9 and 10 are partly elevational, partly sectional views of parts forming the device Fig. 1.

The main body portion of applicants combined plug fuse and fuse integrity indicator, in the spe- 30 cific form as shown in the drawing, is approximately in the form of a hollow cylinder partially walled at one end. It is shown in section in Figs.

2 and 5, and designated 1. This body is formed of insulating material, preferably porcelain. The outer surface is formed into a screw thread 2 for a purpose to be hereinafter described. An orifice 3 is formed in the side Wall for the reception of a pin 4. The partial end wall 5 is formed with a shoulder 6, a bore 7 and a counter bore 8. The 90 cylinder is somewhat enlarged at one end, as at 9, to afford a nger hold, and in order to make this more efficient the surface is roughened or knurled or corrugated as shown in Fig. 1.

A bottom end closure for the body 1 is shown 95 in Fig. 6, and designated 10. This bottom end closure is also made of insulating material, preferably porcelain, and is substantially in the form of a somewhat attened truncated cone having a bottom 11 and a top 12. The member l0 is 160 provided with an annular shoulder 13 which is formed to bear against the lower face l5 of the body portion l. The portion i6 of the bottom end closure 10 is made of a diameter such that it will just snugly fit between the inside surfaces of the wall of the body l. The bottom member l when put in position is held therein by the me tallic shell 17. This metallic shell has its side Walls formed as a screr.r thread which engages with the screw thread 2 upon the body l, and the llo with an indicator.

flange 18 bears against the annular surface 19 of the end member 10 thereby holding the end member 10 firmly in positiomas shown in Fig. 2.

The bottom end closure 10 is formed with a bore 20 into which the combined lterminal contact and button fastener 21 extends. The button fastener 21 is preferably formed with a head 22 on one end which bears in a recess 23 of the bottom end member 10 and is preferably provided at its other end with two prongs 24 and 25 which may be bent over so as to bear against the upper surface 11 of the end member 10 s0 as to hold the button member 21 in place.

The end member 10 is also provided with a duct 26 within which is positioned a conducting strip 27. This conducting strip is provided with an oriced head 28 which surrounds the button member 21, and is positioned between the head 22 and the button member 21 and the bottom end member 10, so that it is thereby held in proper position when the button member is fastened in position by turning the lugs 24 and 25.

A thermal fusible'element 29 has one end attached to the conducting strip 27 and the other end passes through an orifice 30 of body 1 and lies in a slot 31 of body 1 in position to be contacted by the metallic shell 17 when it is placed in position, but to make electrical contact more certain a drop of solder 32 is preferably used to join the two together.

The parts hereinbefore described, assembled in the manner specifled, constitute a desirable and efficient plug fuse. The parts may be made of such dimensions that the whole device is of the dimensions of the ordinary standard plug fuse. When such a plug fuse is inserted in a receptacle, as is usual, and an excess flow of current takes place therethrough the thermal element 29 will fuse and interrupt the continuityof the circuit between the button 21 and metallic shell 17 and so perform the function for which the device is designed.

A plug fuse, such as has been described, is usually provided with a mica window covering the opening or bore 'I in the top or end wall of the body 1 so that inspection may be had of the contents to determine whether or not the thermal element 29 is intact. Such construction has been found to be hightly inefficient in that the mica, as a rule, is not transparent enough to enable a good view ofthe contents of the body 1 to be accurately determined. This diiliculty is greatly increased by the fact that the plug fuses are usually used in situations where the illumination is poor.

In order to obviate the diiiculty with prior plug fuses, applicant proposes to equip the plug fuse of the construction hereinbefore described Applicantsindicator is in the form of an electric lamp connected across the terminals df the plug fuse so that when the thermal element 29 is intact the lamp will not be lighted, butwhen the thermal element 29 has been destroyed the lamp will then be lighted.

The preferred form of construction desired by applicant is shown in the drawing. Applicant preferably provides a tube 33 within which the indicator is to be positioned. This tube is for the purpose of separating the explosion chamber 34' from the indicator. This is desirable but not entirely necessary. The tube 33 is preferably made integral with the body member 10 and is of such dimensions that it fits nicely into the counter bore 8 of the end wall 5 of-the body 1, resting against the shoulder 6.

Applicant prefers a construction of combined indicator and fuse plug such that the indicator may be readily separated and attached to what might be called the plug portion, to the end that the plug portion may be rejected if the thermal element 29 has been destroyed, but the indicator portion retained to be combined with a new plug portion.

Applicants preferred construction of indicator per se is well shown in Fig. 2 wherein a lamp 34 is illustrated. This lamp 34 is preferably of a low potential discharge type.

Applicant prefers to enclose the lamp 34 in the tube 35 and provide the tube with a metallic annulus 36 and a metallic end 37, to one of which one terminal of the lamp is electrically connected, and to the other of which the other terminal of the lamp is electrically connected. Applicant also introduces a resistance 38 between one of the terminals of the lamp and one of the metallic members on the outside of the tube 34. This resistance preferably takes the form of the well known grid leak resistance, and is of a value such that no appreciable current passes through the lamp at any time, that is, the lamp is not, relatively, an appreciable load upon the circuit when the fusible element 29 is intact, and does not provide, relatively, a substantial path for the fiow of current when the fusible element 29 has been destroyed.

In order to make connection to the metallic annulus 36, applicant positions it so that it will make contact with the metallic pin 4, the head of which is conductively attached to the metallic shell 17,'preferably by the same drop of solder `that attaches the end of the thermal member 29.

In order to properly bring pin 4 in contact with annulus 36 an orifice 39 is formed in the tube 3.

The metallic end member 37 not only serves to electrically connect with the button member 21 but also serves to mechanically connect therewith. This is accomplished by forming the members 21 and 37 in an intermating way so that the member 21 snap fastens with the member 37. This is accomplished by forming the ordinary ball and socket snap fastener of the parts, that is, forming a ball upon the member 21, and a socket in thelmember 37. By this device the tube 35 may be inserted within the tube 33, and with a slight push may be firmly fastened in place, but a proper pull will release it so that it may be withdrawn.

If desired, the tube 35 with its associated parts may be of such length that it protrudes slightly if at all beyond the en d wall 5 of the plug fuse, but applicant prefers to allow it to protrude just sufficiently to enable it to be grasped by the ends of the fingers and withdrawn. In order to facilitate this manipulation applicant roughens or knurls the end of the tube 35 as at 40.. Applicant also provides an end wall. 41 in the tube 35 and a bore 42 therethrough through which the illumination of the lamp 34 may be observed. It is also preferable, but not entirely necessary, to provide a transparent disc 43 for closing the opening of the bore 42.

Applicant contemplates the preferred form of his device as being one in which the tube 35 is removable so as to be conveniently placed in another fuse body. If it is not intended to be removed its upper end may be flush with the top surface 5 but whether or not it is to be removed the amount which it has to extend beyond the body 5 is such that the combined fuse and fuse 'I Letters Patent of the UnitedStates, is:

l. A combined plug fuse'and thermal element integrity indicator comprising, incombination: a body portion substantially the form of a hollow cylinder; a threaded metallic shell partially covering the outer curved surface of the body, form? ing one electrical terminal' thereof; another terminal positioned substantially centrally oi one end oi the body; means to support the second named terminal in a fixed relation to the body; a thermal element connecting the two terminals and positioned largely within the body; a lamp provided with two terminals; means for connecting the terminals of the lamp to the terminals on the body; said body formed with an end wall formed with a bore therein positioned axialiy in line with the centrally positioned terminal; a tube supporting and enclosing the lamp and positioned axially of the body and passing through the said bore.

2. Aplug fuse having in combination: an outer cylindrical, screw threaded metallic shell oontact; a metallic contact positioned axially of the shell; a thermal element connecting the contacts; an elongated lamp having two terminals; means supporting and enclosing the lamp and positioning the lamp with its longitudinal axis axially of the metallic shell, said means removable bodily `and including means fonforming electrical connections 'to the terminals; and means for supporting all parts in fixed position.

3. In a combined plug fuse and fuse integrity indicator, in combination: a terminal, said terminal formed with one member of an intermating snap fastener; a tube having its end wall vwhen the thermal element is destroyed the lamp will be lighted. f

4. In a combined plug fuse and thermal element integrity indicator, in combination: a subi stantially cylindrical hollow body; a terminal upon the curved surface of the body; a terminal 'positioned centrally of the end of the body; an

elongated lamp positioned with its longitudinal axis substantially axially of the body; a thermal element connecting the terminals of the body; and means supporting 'and enclosing the lamp for positioning the lamp, and for bodily withdrawal -carrying the lamp from its position'away from the body, and maintaining the lamp removably' attached to the body, and also for electrically connecting the lamp to the terminals, whereby the lampmay be positioned in the body or removed for positioning in another like body.

5. In a combined plug fuse and thermal element integrity indicator, in combination: a substantially cylindrical hollow body provided with a screw threaded shell partially covering its outer curved periphery, said body being provided with an end wall formed with a bore and a shoulder; an end closure for the other end of the body adapted to be held in place by the said shell, said end closure formed with an orifice and a duct; a button terminal formed with an enlarged head at its outer end and one member oi a two part snap iitting kon its inner end positioned inl the said orice; an electrically conducting metallic strip positioned under the head of the button member and passing through the said duct into the hollow interior of the body; a thermal element connecting the last mentioned strip with the outer shell; a tube extending centrally of the body and positioned between the end closure and the end wall and forming a space separate from the space in which the thermal element is positioned; an electric lamp positioned within the `the button terminal and adapted to engage therewith and thereby be removably secured thereto;

a conductor extending from one terminal of the lamp tovthe metallic end wall of the tube;a

metallic member` positioned upon the outside of the tube surrounding the lamp; a conductor connecting the other terminal of the lamp with the said metallic member upon the outside of the said tube; a resistance interposed in thelast mentioned conductor; andv a metallic conductor frictionally engaging the metallic member upon the outside of the lamp tube and `extending to the metallic shell.

Y' FRANK C. LA MAR.

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tube; -a tube surrounding the lamp and fitting 

